The invention relates to an optical reflectance probe system for the illumination of a sample material and detection of reflected light.
Optical reflectance measurements are commonly used for the analysis of materials. In a typical optical reflectance system, light is shown upon the material to be analyzed. An optical detector/measurement instrument gathers some of the light reflected off of the material and measures the intensity of the light either at specific wavelengths or across a spectral range yielding a measurement of intensity versus wavelength.
Materials can be analyzed in this way for the presence of certain constituents, the amount of these constituents, and the uniformity of these constituents throughout the consignment of the material. Specific uses include measurement of blend uniformity in pharmaceutical products, water or other solvent content in pharmaceutical products, measurement of protein, carbohydrates and water in agricultural products, and the presence of foreign material in an otherwise homogeneous material such as flour. Other applications include paint matching, quality control for paper, textiles, packaging, food, pharmaceuticals and cosmetics.
Typically, an arrangement of a light source, lenses and mirrors are used to align and project the illumination from the light source through a viewport window onto the sample material. Then additional lenses and mirrors are used to capture the light reflected from the sample material and guide it to the optical detector/measurement instrument. Optical fibers are also commonly used to guide the illumination light to the sample and/or optical pickup fibers to capture and guide the reflected light from the sample material back to the optical detector/measurement instrument. Common light sources include incandescent and particularly tungsten-halogen lamps. Common optical detector/measurement instruments include photometers, monochronometers and optical spectrographs.
Optical reflectance measurement systems require calibration. Calibration includes the use of reflectance standards including white references, references with known spectral signatures, spectral line sources, transmissive filters, and shutters. Calibration generally takes place during manufacture of the optical detector/measurement instrument, and commonly again after the system components are integrated. Calibration of the system can change due to vibration, temperature change or other conditions, so it is common to recalibrate periodically to ensure the system is performing within a required accuracy. In certain applications, such as the production of pharmaceutical products, there are government regulations requiring periodic verification of performance, and again, requiring the use of these calibration standards.
Current optical reflectance measurement systems require that some or all of these standards be employed by an operator dismounting the probe and manually introducing these references for the system to sample. This can be a cumbersome and time consuming task, as the system may be mounted at a point generally inaccessible. The unit could easily be damaged during the removal, or during reinstallation, requiring the system to be repaired, recalibrated, or worse, go unnoticed where data generated by the system is relied upon to produce safe and effective product.
In process control or quality control applications, optical reflectance measurement systems are required to be adjacent to the sample material being measured. Where the sample material is contained in a chamber, such as a vacuum chamber, mixer, blender or environmental chamber, the optical reflectance probe must view the sample material through a viewport window. This window must withstand pressures, abrasion, chemical attack, and provide a seal between the probe and the chamber interior, while providing a clear optical path for the probe to view. Further, mounts for the optical reflectance measurement system must be provided to hold the probe in reference to the window to view the sample material within the chamber.
Current window and mount systems employ a flat viewport window and a series of mounting brackets. The viewport window reflects some of the illuminant light from the probe back into the probes collecting optics, thus distorting the reflectance measurement. Anti-reflection coatings on the window reduce but do not eliminate this back reflection. Further, these coatings cannot be applied to the inner surface of the window because some of the coating may abrade off, contaminating the material, and generally cannot withstand chemical attack and other environmental conditions. Other means to reduce effects caused by this back reflection require complicated optical schemes including collimation and focusing optics. The mounting brackets are generally custom for the particular chamber and optical reflectance probe being employed, and must be designed special for each application. Further, due to constraints placed by chamber geometry and the requirements of bracket position and orientation to the window, placement of the window at a desired viewing position may not be possible for certain applications.